Group III nitride semiconductors, a typical example of which is GaN, have a dielectric breakdown field strength that is approximately 10 times greater than that of conventional Si, and their saturation drift velocity is approximately 3 times greater than that of conventional Si which is excellent from the viewpoint of mobility. Because of this, group III nitride semiconductors have attracted attention as a semiconductor material for high frequency—high output devices. FIG. 9 shows a cross-sectional structure of a conventional semiconductor device 2 that uses a nitride semiconductor. A buffer layer 22 that is formed from a nitride semiconductor material is formed on a substrate 21. A semiconductor layer 23 that is formed from a nitride semiconductor material such as GaN is formed on the buffer layer 22.
A semiconductor layer 24 that is formed from a nitride semiconductor material such as AlXGa1-XN (0<X≦1) is formed on the semiconductor layer 23. Electrodes 25 and 26 are formed on a primary surface 200 of the semiconductor layer 24. A polarization charge is generated by the spontaneous polarization of the nitride semiconductor on an interface 201 at a boundary between the semiconductor layer 23 and the semiconductor layer 24. Furthermore, piezoelectric polarization (also known as piezoelectric field polarization) is generated in the vicinity of the interface 201 by distortion that is generated by the difference between the lattice constant of the nitride semiconductor of the semiconductor layer 23 and the lattice constant of the nitride semiconductor of the semiconductor layer 24. As a result, a two-dimensional carrier (also known as a two-dimensional electron gas layer or a two-dimensional hole gas layer) 202 is generated based on these polarization charges in the vicinity of the interface between the semiconductor layer 23 and the semiconductor layer 24.
This distortion is caused by mechanical tensile stress that is generated in the semiconductor layer 24 as a result of the difference between the size of the lattice of the semiconductor layer 23 and the size of the lattice of the semiconductor layer 24, and this distortion contributes to the piezoelectric polarization. Because the semiconductor layer 23 and the semiconductor layer 24 have considerable band gap energy, a high density two-dimensional carrier is generated in the vicinity of the interface 201. Note that a semiconductor having a similar structure to that described above is described in Japanese Unexamined Patent Application, First Publication No. 2003-100778.
However, in a conventional semiconductor device that uses a nitride semiconductor material, the internal resistance inside the semiconductor between the electrodes that are provided on the semiconductor layers and the two-dimensional carrier is comparatively large and the problem has existed that it has not been possible to sufficiently lower the forward voltage. Moreover, parasitic capacitance is generated between the electrodes and the two-dimensional carrier and the problem has also existed that this has also affected the high frequency characteristics of the semiconductor device.